Transmission control in two-way signaling systems



[w M w EH2 m Aug. 4, 1915f). B. G. BJORNSON 2,049,941 TRANSMISSIONCONTROL IN TWO-WAY'SIGNALING SYSTEMS Filed 001;. 51, 1935 POLAR/ZEDHIIJI FIG. 3

J MTECTOR -POLARIZED ll' 59 44 9 L 49 1a fik POLAR/ZED 6'0 T 2 POLARIZEDINVENTOR B. G. BJORNSON A TTORNEV Patented Aug. 4, 1936 STATES FATE riceTRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Application October31, 1935, Serial No. 47,579

I 6Claims. (01. 179-5170) 7 This invention relates to two-way signalingsystems and particularly to the signal-controlled switching circuitsemployed for directionally controlling signal transmission in suchsystems.

An object'of the invention is to improve the operationof such circuits.

To obtain proper operation of long two-way signaling systems, such asfour-wire toll telephone systems, it has been found necessary in the 10past to make use of voice-operated switching apparatus, so-called echosuppressors or anti-singing devices, for inserting a suitable loss inthe signal path for one direction when signal transmission is takingplace over the signal path for the opposite direction so as to preventechoes or reflected currents from beingtransmitted back to thetransmitting end of the system and causing a disturbance or'singing.Such suppressors usually include an amplifier-rectifier circuitconnected to the signal path for each direction, which is responsive tosignal transmission therein to insert a suitable amount of loss in thesignal path for the opposite direction.

Certain suppressors of the prior art are of the type which operate tocompletely disable the' echo path orto insert a large loss of fixedvalue therein. Another type of suppressor in the prior art whenoperated'inserts a loss in the echo path which, above the operate point,is directly proportional to the amplitude level of the waves in theother path at the input to the suppressor. With this suppressor, theecho path is never completely blocked. Loss is inserted in the echo pathto reduce the echo below a certain predetermined level. Among theadvantages of this type of suppressor may be mentioned easier break-ins;the avoidance of complete blocking of the circuit when the noise isabove the minimum operate point of the suppressor; and the prevention ofintermittent blocking of the circuit by the echo suppressor when thegain is raised beyond the singing point. The disadvantage for the lossertype of circuit is that if the above advantages are retained, the echoesfrom the final parts of the syllables are objectionable.

' -In accordance with the present invention, the two above-describedtypes of echo suppressors are combined in such manner as to realize theadvantages of both while minimizing their disadvantages. The circuits ofthe invention are arranged to take advantage of the fact that the partof the echo which is returned to the talker'before the end of his speechsyllable, or just after, requires less attenuation than the final partof the echo, due to the desensitization of the ear of the talker byspeech during the initial time interval.

' In one embodiment of the invention, acontrol circuit at or near eachterminal of the four-wire circuit is responsive to incoming waves above5 the operate level to insert a small loss in the outgoing repeatingpath thereat, the value of which is directly proportional to the controlcircuit input power, and a second control circuit at or near eachterminal is responsive to outgoing speech 10 waves to insert a largeloss in the incoming path. The circuits are arranged so that the smallloss attenuates the echo of the initial part of each speech syllable andthe large loss suppresses the echo of the final part of each syllable.15

The objects and advantages of the invention will be better understoodfrom the following detailed description thereof when read in connectionwith the accompanying drawing inwhich:

Fig. 1 shows diagrammatically a four-wire toll telephone system equippedwith wave-operated 20 transmission control apparatus which may bedirection of transmission. An arrow within the triangular box indicatesthat the transmission element is variable, and to show that the loss orgain of such a variable transmission element is to be varied in responseto the level of waves impressed on an associated wave-operated controldevice (amplifier-detector), an arrowhead points 40 from the controldevice towards the variable element. A normal make in a transmissionpath is indicated by contacting arrowheads. An arrow directed at a makepoint indicates that the path will be disabled at that point by theassociat- 45 ed wave-operated device.

The four-wire telephone circuit of Fig. 1 comprises a one-waytransmission path EA including the one-way amplifying devices A1 and A2for repeating telephonic currents in the direction from west to eastbetween the two-way circuit leading to a west station SW and the two-waycircuit leading to the east station SE, and the oneway transmission pathWA including the one-way amplifying devices A3 and A4, for repeatingtele 5 phonic currents in the direction from east to west between thetwo-way circuit associated with the east station SE and the two-waycircuit associated with the west station SW. In any suitable manner, theoppositely directed one-way transmission paths EA and WA may beconnected in substantially conjugate relation with each other and inenergy transmitting relation with the two-way circuits leading tostations SW and SE, respectively, for example as indicated,

by hybrid coil transformers H1 and H2 and asso' Y ciated balancingnetworks N1 and N2.

Connected to the path EA at some point I near its input is the input ofa voice-operated device or echo suppressor ESi which operates inresponse to impressed voice signal waves from the path EA to disable thepath WA at some point 2 near its output. Connected to the path EA atsome point 3 near its output is the input of a proportional losserconsisting of a wave-operated control device (amplifier-detector) 4 anda loss device L2, such as a variable gain amplifier, controlled thereby,which operates in response to impressed waves from the path EA aboveacertain critical amplitude to insert a proportional transmission lossin the input of the path WA. Connected to the path WA at a point 5 nearits input but in theoutput of the loss device L2 is the input of thevoice-operated control device or echo suppressor ES2, which operates inresponse to impressed voice signal waves from the path WA to disable thepath EA at a point 6 near the output thereof but in front of the pointof connection 3 of the proportional losser thereto. Similarly, connectedto the path WA at a point I near its output but beyond the disablingpoint 2 therein is the input of another proportional losser circuitcomprising the wave-operated control device (amplifier-detector) 8 andthe loss device L1 controlled thereby, which operates in response to.impressed waves from the path WA above a certain critical amplitudeto'insert a proportional transmission loss in the path EA in front ofthe point of connection I of the echo suppressor ESl thereto.

The details of a suitable proportional losser which may be used in thesystem of Fig. 1 are illustrated in Fig. 2 of my Patent 1,840,015 issuedJanuary 5, 1932, in which the proportional loss is inserted in onerepeating path by suitably con- 7 trolling the bias on the grid of avacuum tube amplifier in that path in accordance with the amplitudelevel of waves in the other path, under control of an amplifier-detectorcircuit connected thereto. Other arrangements of proportional losserswhich may be used in the system of Fig. 2 are illustrated in A. B. ClarkPatent No. 1,821,928, issued September 8, 1931, and in H. C. SilentPatent No. 1,745,457, issued February 4, 1930.

A preferred construction of the echo suppressors ES1 and E332 employedin the system of Fig. 1 is shown in Fig. 2 as indicated. The suppressorincludes an amplifier 9, the output of which is coupled by a transformerto the input of a three electrode detector tube I0, the plate circuit ofwhich includes in series with plate battery I3, the windings ofmechanical relays II and I2, which windings are shunted by a condenser63. The terminals of the winding of a third relay I4 are connectedthrough the normally closed armature and contact of relay II in serieswith the resistances I6 and IT. A condenser I5 is connected in shuntwith resistance I! through the normally closed armature and back contactof relay I2. When the relay I2 operates to shift its ing of a fourthrelay 2| from battery 2!! through series resistances 22 and 23 andground, causing the operation of relay 2| to open its normally closedarmature and contact.

The junction of series resistances 22 and 23 is connected to groundthrough condenser 24 for a purpose which will be brought out later.

The operation of the system illustrated by Fig. 1 will now be describedusing the circuit shown in Fig. 2 for the echo suppressor ESi and forthe echo suppressor ESz.

When the circuit of Fig. 2 is used as the echo suppressor ES], in thesystem of Fig. 1, the input of the amplifier 9 is connected across thepath EA at the point I and the normally closed armature and contact ofrelay 2I are arranged in any suitable manner to render the path WAoperative at the point 2, and when the armature and contact areseparated byoperation of the relay, to disable that path or to insert alarge disabling loss therein at the point 2. Similarly, when the circuitof Fig. 2 is used for the echo suppressor .ESz in the system of Fig. 1,the input of the amplifier 9 would be connected across the path WA atthe point 5, and the armature and contact of relay 2| would be arrangednormally to render the path EA operative at the point B and to insert alarge disabling loss at that point in the path When relay 2I operates toseparate its armature and contact.

It will be assumed that speech waves for transmission from west to eastare received by the four-wire repeating circuit over the two-way lineconnected to the west station SW from a west subscriber and, at thattime no speech waves have as yet been received at the east terminal ofthe four-wire circuit over the two-way line connected to the eaststation SE.

The received speech waves in the path EA will be amplified by theamplifier A1 and will pass through the loss device L1 of theproportional losser circuit arrangement located near the west terminalof the four-wire circuit. If the noise level at the point I in the pathWA at that instant is sufiiciently high to cause operation of theamplifier-detector device 8, the loss device L1 is inserting in the pathEA a small amount of loss which is directly proportional to the level ofthe noise currents impressed upon the amplifierdetector 8. The-circuitsare arranged so'that the amount of this small amount of loss at anyinstant is just sufficient to prevent operation of the echo suppressorES1 connected to the path EA at the point I by the noise waves alone inthe latter path. The main portion of wests speech waves slightlyattenuatedby the loss device L1 will be transmitted out over'the path EAtowards the east terminal of the four-wire circuit; a small portion,however, will be diverted to the suppressor ES1 at'thepoint I and willcause its operation to disable the path WA at the point 2 in a-mannerwhich will be later described in detail in connection with Fig. 2. a

At; the east endof theiour-wirecircuitgthe 'main portion of the incomingspeech waves will be amplified by the amplifier A2 and impressed by thehybrid coil H2 on the associated two-way line overwhich itwill betransmitted to the listening subscriber at the east station SE. A smallporthrough resistan'ces'IB and-11. The relay I2,how

tion of theincoming speech waves (wests) will be diverted from the pathEA into the proportional losser 'circuit connected at the point 3.

Since the amplitudes of these waves will exceed ;the critical valuewhich will cause operation of of wests speech entering the input of thepath WA through the hybrid coil H2 and transmitted westward thereover tothe two-way circuit associated wtih the .west station SW so that it willnot be heard by the speaker at that station. The amount of this loss maybe made quite small inasmuch as experiments have shown that during theinitial portion of each spoken syllable, the

speaker's ear is highly desensitized and only re-' gains its normalsensitivity gradually. The loss inserted by the loss device L2 near theinput of the path WA is also such as to prevent false operation of theecho suppressor ESz connected at the point 5 in the path by the echo,which suppressor if operated falsely would disable the path EA at thepoint 6 in its output and thus thereafter prevent wests speech currentsfrom being transmitted to the east subscriber associated with stationSE.

Because of the amount of additional attenuation undergone by the echocurrents in transmission over the path WA from the output of the lossdevice L2 to the point I near the west terminal of the four-wirecircuit, the echo waves impressed on the control device 8 of theproportional losser circuit connected at that point will be of such lowlevel that they will either not operate that control'device or, if theydo operate it, the amount of loss inserted in the input of the path EAby the loss device L1 and the time for which .it is inserted will be sosmall as not to degrade the subsequent speech transmission over the pathEA appreciably.

The above description explains how the arrangement of Fig. 1 functionsto prevent undesirable echo effects due to the initial portion of eachsyllable. The operation of the circuits of Fig. 2 in suppressing echoesof speech due to the final portion of each syllable which arrive at thetransmitting station after the reduction in hearing sensitivity causedby the syllable has been reduced, will now be described.

As stated previously, a small portion of wests speech waves at the westterminal of the fourwire circuit is diverted at the point I into theecho suppressor ES1. Referring to Fig. 2, this portion will be amplifiedby the amplifier 9 and impressed on the input circuitof the tube I0which is biased by a grid battery I52 to act as a detector. Theimpressed speech impulse will overcome the normal negative grid bias onthe tube and cause plate current'to flow in the plate circuit thereoffrom battery I3 through the windings of relays II and i2 in series andthe platefilament space path of the tube. The alternating currentcomponents of this plate current will be kept out of the relaywindingsby condenser 63.

Relay II is designed to operate in response to the detected speechsyllable above a given amplitude to open its armature and contact,breaking the connection aroundthe winding of relay I4 ever, is designedto operate when, and if, the amplitude of the detected speech syllablerises a given amount, say 3 decibels, above the amplitude which" willcause operation of relay II. If the amplitude of the detected speechimpulse remains below this critical value, the relay will not operateand the path WA will not be disabled at the point 2. If, however, thelevel of the detected speechcurrents is sufilciently high to supply theproper energizing current to the winding of relay I2, that relay willoperate and shift its armature from the back contact to the frontcontact, thus disconnecting condenser I5 from resistance I1 and closinga circuit for that condenser from battery I8 through series resistanceI9 and the back contact and armature of relay I2.

If the detected speech impulse is very short, relay I2 is operated forsuch a short time that the charge accumulated on condenser I5 frombattery I8 will be insuflicient to operate relay I4 through resistanceI6 and the closed armatures and contacts of relays II and I2 when theserelays release. Therefore, on short impulses relay 2I will not beoperated to disable the path \VA. On longer speech impulses, however,the condenser I5 will store up a suflicient charge to opto cause asubstantial time interval between the 3 release of relays I2 and II,most of the charge on condenser I5 will have been dissipated inresistance IT and relay I4 will not operate. The time constant of thecircuit just described by suitable selection of the values of thecircuit elements is made such as to provide for the operation of 'relayI4 if the received speech signal impulse is of a predetermined magnitudeand duration and decays with a predetermined rapidity.

When relay I4 operates, an energizing circuit is closed for relay 2 I,from battery 20 through the closed armature and contact of relay I4,series resistances 23 and 22 and thewinding of relay 2 I. Relay 2| thenoperates to open its normally closed armature and contact so astodisable the path WA or to insert a large transmission loss therein atthe point. 2.

When relay I4 releases after the discharge current from condenser I5ceases to fiow through its winding, the energizing circuit for relay ZIfrom battery 20 through the closed armature and contact of relay I4 isbroken, but the relay 2I does not release immediately but will remainoperated to maintain the path WA disabled for the additional time whichit takes condenser 24 to be charged up from battery 20 through thewinding of relay 2I and resistance 22. This hang-over time interval ismade long enough by suitable selection of the time constants of thecircuit to insure that the loss inserted at point 2 in path WA will bemaintained until all echo waves of the controlling speech waves havereached point 2 and have been dissipated in the disabling loss thereat.H

Since practically all speech syllables retain a substantially constantamplitude for a short time, and since their rate of decay falls withindefinite limits, the proper time constants of the circuit may bedetermined experimentally which will result in the operation of relay 2Iand the consequent insertion of a disabling loss in the path WA onpractically every speech syllable, which loss will be efiective to soattenuate echoes due to the final part of each syllable returned to thetalker at station SW during the period when his ear has recovered itsnormal sensitivity, that they will not be troublesome. Narrow peaks ofnoise and slow variation in the noise just reaching above the operatepoint of the suppressor, because of the time relation just described,will notxcause operation of the disabling relay 2| The requirements foroperation (1) that the length of the speech impulse must exceed a givenvalue and (2) that the amplitude near the operating point must fall offa definite amount in a given time, can be eliminated by omittingresistance I1 and making resistance I9 very small,

Now, if the east subscriber associated with the station SE starts totalk while the west subscriber associated with the station SW is stilltalking, the formers speech currents will be impressed by hybrid coil H2on the input of the path WA and after amplification in the amplifier Aswill pass through the loss device L2 to the input of the suppressor ES2connected at the point 5. As the loss device L2 is controlled by westsspeech currents to insert a proportional loss in the path, easts speechcurrents will be considerably attenuated thereby. However, if east talksloudly enough, his speech currents will operate the echo suppressor E82to disable the path EA at the point 6.

A portion of these speech currents with the attenuation undergone inpassing through the loss device L2 will be transmitted out over the pathWA to the west terminal of the four-wire circuit. At any pause by westlong enough to permit release of the echo suppressor ES1 to render thepath WA operative at the point 2, easts speech currents will get throughand after being amplified by the amplifier A4 will be im pressed by thehybrid coil H1 on the two-wave line leading to the west station SW overwhich it will be transmitted to the west subscriber indicating to himthat east wishes to break in. If both parties then continue to talk,neither will be able to hear the other because of the simultaneousoperation of the echo suppressors E81 and ESz by the voice. currents ofwest and east, respectively. It is to be noted that since echosuppressors E81 and E82 do not stay continuously operated during thetransmission of each syllable, the total time they are held operated ismuch shorter than with other types of echo suppressors. However, if hewishes, the west subscriber may cease talking and allow the eastsubscriber to obtain control of the switching circuit in a mannersimilar to that which has been described for wests speech currents forthe opposite direction of transmission.

As the echo suppressors E81 and ES2 are assumed to be identical, thedetailed operation of the system for the case where the east subscriberis talking and the west subscriber is silent need not be given here.

In the above discussion it has been assumed that the east subscribertalked loudly enough to operate ESz. However, it is more probable toexpect that east will not be able to operate ESz if the west subscribertalks continuously. But, part of easts speech, in spite of the loss inL2, can be heard by West if the suppressor at 2 is lifted. Since ESi isnot continuously operated during speech syllables this condition isfrequently satisfied, and, therefore, easthas beensupplied with'extrafacility for letting west know that he (east) is trying to break in.

Fig- 3. shows an alternative circuit which may be used for the echosuppressors ES1 and ESz in the system of Fig. l. 5

As indicated the system of Fig. 3 comprises in order a vario-amplifier,a detector and a relay circuit. The vario-amplifier comprises athreeelectrode vacuum tube amplifier tube 25, the input circuit of whichin the system of Fig. 1 for 10 the case of the echo suppressor ESi wouldbe connected across the path EA at the point I by the input transformer26, and for the case of the echo suppressor ESz would be connectedacross the path WA at the point 5 by that trans- 15 former.

The grid-cathode circuit of the amplifier tube 25 comprises in seriesthe secondary'winding of input transformer 26, the negative grid biasingbattery 34, and a circuit comprising theresistance 35 and the condenser36 in parallel. The plate circuit of the vario-amplifier tube 25comprises in series the battery 30, the resistance 29 and the primarywinding of the transformer 28. The primary winding of a transformer 32is connected 25 across the resistance 29 in the plate circuit ofamplifier tube 25, and the secondary winding of that transformer isconnected across the resistance 35 in the grid circuit of the amplifiertube and the copper-oxide rectifier 38 in series. 30

The output of the amplifier tube 25 is coupled to the input of thethree-electrode detector tube 4| by the transformer 28, the grid-cathodecircuit of the detector tube including the negative grid-biasing battery40. The plate circuit of the 35 detector tube 4| includes in seriesbetween the filament and plate of the tube the plate battery 42 and thewinding of a mechanical relay 49. The winding of the relay 43 isshunted. by the condenser 44. 40

The windings of two otherrrelays 46 and 41 are connected in seriesthrough the series condenser 45 and the normally closed armature andright-hand contact of relay 43. When relay 43 operates to shift itsarmature from the righthand to the left-hand contact, the connection ofthe windings of relays 46 and 41 through condenser 45 is broken and thebattery 48 is connected across the condenser '45 and series resistance49.

The winding of a fourth mechanical relay 5| is normally energized bycurrent from the battery 58 through series resistances 59 and 60, thenormally closed armature and contact of relay 4'! and ground. Thearmature and contact of 55 relay 46 are normally open and when closed byoperation of that relay complete an energization circuit for the windingof a fifth mechanical relay 5!) from battery 52 through seriesresistances 53 and 54, the closed armature and contact of relay 60 '46and ground. The condenser 51 is connected across the winding of relay 5|and resistance 59 in series and serves to make that relay slowoperatingand slow-releasing, and a condenser-55 is connected between the junctionof series're- 65 sistances 53 and 54 and ground'for a purpose to bebrought out later.

point 2 in the case of the echo suppressorES1 and to disable the path EAat the point 6 in the case of the echo suppressor 152.

The armature and contacts of relay 5| are normally maintained open dueto the normal energization of the winding of that relay by battery 53.When the relay 5| releases to close its armature and contact,ashort-circuiting connection 62 including the series resistance 6| isconnected across the parallel condenser 36 and resistance 35, and acrossthe secondary winding of transformer 32 in series with copper-oxiderectifier 38.

The echo suppressor arrangement of Fig. 3 is designedto operate todisable the echo path when the syllable amplitude of the impressedsignal impulse has fallen a definite amount below its 7 ordinaryconversation, complications develop, the

effects of which are nullified in the circuit of Fig. 3. The circuitoperates as follows:

In the normal condition of the circuit, that is,

when no speech signal is being received, there is no charge on thecondenser 36 in the grid circuit of the tube 25 and the vario-amplifier,therefore, is at maximum gain. The tube 25 is preferably a variable atube. Now, if a speech signal impulse is received from the associatedrepeating path, this signal is impressed by the transformer 26 on thegrid circuit of the vario-amplifier tube 25 and will cause current toflow in the plate circuit of the tube through the resistance 29 causinga voltage drop corresponding to the value of the current across thatresistance. This voltage is fed back into the grid circuit of tube 25through the transformer 32, the copper-oxide rectifier 38 in series withits secondary winding, and the parallel condenser 36 and resistance 35in the grid circuit of tube 25, producing on the condenser 36 acorresponding charge. This charge adds to the negative bias on the gridof the tube and thus efiectively reduces the gain of the amplifier inproportion to the increase in the amplitude level of the applied signalimpulse, so that its output level remains substantially constant.

By suitable selection of the circuit constants of the amplifier and ofthe feed-back control circuit, the maximum output level of thevario-amplifier tube 25 is maintained a few decibels above the operatepoint of the detector tube 4|.

The amplified signal impulse in the output of the amplifier 25 isimpressed by transformer 28 on the input of the detector tube 4|,overcomes the normal negative grid bias thereon supplied by grid battery46, and causes plate current to flow in the plate circuit of thedetector tube 4| through the winding of relay 43. The condenser 44 keepsthe alternating current component of the detected signal impulse out ofthe relay winding and thus smooths out the energizing current in thatWinding. Relay 43 then operates to shift its armature from theright-hand contact to the left-hand contact, disconnecting the'condenser45 from in shunt of the windings of relays 46 and. 41, and connecting itacross the battery 48 in series with resistance 49 so that the condenseris charged up to the value of the battery 48.

When the signal impulse applied to the amplifier 25 begins to decay,that is, to fall off after reaching its maximum level, the gain of thevarioam'plifier 25 is held practically fixed, since the resistance 35 inshunt with the condenser 36 in the grid circuit of the amplifier tube ismade peak of the next signal impulse.

large. 'At the end of the' signalimpuls'e, relay 43-will release and thearmature thereof willl'eave the'left-hand contact and return to therighthand contact, thereby removing battery '48 from f condenser 45 andreconnecting condenser45 in 5 shunt with the windings of relays Miami4'! in series. 7

The condenser 45 will now discharge through the windings of relays 46and '41 causing the operation of these-relays. When relay46 operates itsarmature and contact close completing an energization circuit for relay56 from battery 52jj through the series resistances 53 and" 54 andground. Relay 50 will then operate to open its normally closed armatureand contactthusinserting a large disabling loss in 'the'e'cho path WA orEA depending onwhether the circuit is used in echo suppressor ESr'orecho suppressor ESz. Due to the delay in the operation of the suppressoruntil the syllabic amplitude has fallen a definite amount below itsmaximum value, the disabling loss is inserted into the echo pathat themost, effective time, that is, only in time to attenuate the last partofthe echo of each speech syllable which will arrive at the talkers end of2 the system just about the time when the talkers ear has regained itsnormalsensitivity. Due to the insertion of this large loss the returningecho Will be of such small amplitude as not to be troublesome. Aspreviously described in connec: tion with'the system of Fig. 1, theinitial part of the speech echo which arrives at the talkers end of thesystem while his ear is still in th e highly desensitized condition dueto his own speech is attenuated only just the amount necessary toprevent it being troublesome, by the proportional losser circuit at thereceiving end of the fourwire circuit.

The relay 46 releases when the condenser 45 has been completelydischarged through its Winding to reopen its armature and contactbreaking the normal energizing circuit for relay 56 from battery 52, butrelay 50 does not immediately re-' lease being maintainedenergized foran additional hang-over interval while condenser 55,. which wasdischarged when relay 46 operated is being charged up from battery 52through the winding of relay 5!], resistance 53 and ground. By suitableselection of the circuits associated with relay 56, this hang-over timeinterval is made sufficient to insure that all echoes of the COHtI'OI-rling speech syllable have reached the disabling point in the echopathand are dissipated thereat before relay 5!] releases to remove thelarge loss.

As stated above, when the signal impulse begins to decay, the gain ofthe vario-repeater is held practically fixed, since the resistance inshunt with the condenser 36 in its grid circuit is quite large. When therelay 43 releases, it is necessary to increase the gain of thevario-repeater so that the relay 46 can operate at least on the The gainmust not be returned to normal so quickly that the relay 43 will operateagain on the tail end of the same signal impulse. The reset relay 5|under control of the relay 4! performs this function. After relay 43releases, the winding of relay 41 is energized by the discharge currentof con-. denser 45 along with the winding of relay 46 asv describedabove and will operate to break its armature and contact thus breakingthe normal energization circuit for the winding of relay M, from battery58 through series resistances 59 and 60 and ground. The reset relay 5|is a slowoperate and slow-release relay due to the efiect and resistance59. When the energization cir cuit for the winding of relay 5| frombattery 58 is broken, condenser 59 will discharge through the relaywinding and resistance 59 maintaining the relay operated for'anadditional interval of time after which the relay releases. When relay5| releases, its armature and contact close the circuit 62 including theseries resistance 6| across the condenser 36 and parallel resistance 30causcondition when condenser 45'has completely discharged through thewinding of the relay, the contact and armature of the reset relay 5|will be maintained in the open condition for the time that it takes thecondenser 51 to be charged up from battery 58.

This simple method of insuring operation of the suppressor on eachspeech syllable will operate satisfactorily in practice because inpractically every case the longest decay period of a speech syllable isshorter than the shortest time interval from the beginning of the decayof syllable to the beginning of the decay of a following speechsyllable.

In the systems which have just been described, the location of theproportional losser should be as near the receiving end of the four-wirecircuit as possible so as not to insert loss before it is necessary.-The echo suppressors should be located as near the transmitting end ofthe fourwire circuit as possible, so as not to insert high loss in theecho path until the talker has just finished saying a syllable. Theproportional lossers should be made more sensitive than the echosuppressors in order to insure their, operation first.

The echo suppressor circuits of the invention described above. have beenfound to be an improvement over the ordinary proportional loss type ofsuppressor in that' the initial sensitivity is the same as for thetypeof suppressor operating to completely disable the echo path, forequivalentecho suppression, and in that no hangover is associated withthe losser circuit which tends to make the loss proportional to thenoise peaks. They are an improvement over the type of suppressor onlyoperating-to entirely disable the echo path, in that (1) the systemallows the listener to get some speech through to the talker more easilythan the latter type of suppressor, and (2) in that transmissionlock-outs characteristic of the latter type of suppressor when bothsubscribers start talking at about the same time, are prevented, for inthe present systems the amplitude of singing current is limited and thecircuit is never completely blocked.

Various modifications of the circuits of the invention which have beenillustrated and described within the spirit and scope of the inventionwill occur to those skilled in the art. The invention-is only to belimited in accordance with the appended claims.

What is claimed is:

1. In a two-Way telephone system comprising at least near the terminalsthereof two one-way f the condenser 51 in shunt with its windingcircuits for transmitting the :speech signals'in opposite directions,acontrol deviceconnected to the incoming one-way circuit 'near oneterminal of the system, responsive'to the incoming signals to insert intheoutgoing one-way circuit near that terminalfla loss which is directlyproportional to the amplitude level of the impressed signals above acertain minimum level, a second control device connected to the outgoingone-Way circuit near the other terminal, which is operative at or nearthe end of each transmitted speech syllable to insert a loss of largevalue in the incoming one-Way circuit near said other terminal.

2. The system of claim 1, in which said second control device operatesto insert said loss of large value in the incoming one-way circuit nearsaid other terminal when the syllable amplitude of outgoing speechsignals has fallen a definite amount below its maximum value.

3. The system of claim 1, in which said second device controls a wavedetector having its input connected to the outgoing circuit, a chain ofmechanical relays controlling the insertion of said large loss and beingoperated in sequence in response to the rectified signals in the outputof said detector, the time constants of said relays being selected sothat the last relay in the chain operates to insert said loss at thedesired time.

4. The system of claim 1, in which said second control device comprisesan amplifier for amplifying the signals received from the outgoingoneway circuit near said other terminal, a detector for rectifying theamplified signals, a chain of mechanical relays operating in sequence inresponse to the rectified energy, the last relay in the chain operatingto insert said large loss, means also responsive to the rectified energyto so adjust the gain of said amplifier that all rectified speechsyllables are reduced to the same energy level, the time constants ofthe relays in said chain being so selected that said large loss isinserted in the incoming one-way circuit at the proper time.

5. In a two-Way telephone transmission system, a four-Wire repeatingcircuit for repeating speech signals'in opposite directions between twotwoway lines, a voice-operated switching device connected to each sideof the four-wire circuit near its output and responsive to incomingspeech signals to insert in the input of the other side of the circuit aloss which is directly proportional to the power input to said deviceabove the operate value, and a second voice-operative device connectedto each side of said four-wire circuit near its input beyond the pointwhere the loss is inserted by said first device connected to the otherside of the four-wire circuit, said second device operating at or nearthe end of each transmitted speech syllable to insert a loss of largevalue in the output of said other side in front of the point where saidfirst device is connected.

6. The system of claim 1, in which the firstmentioned control deviceoperates to insert said proportional loss in the outgoing one-waycircuit near said one terminal quickly in response to the initialportion of each syllable of the impressed speech signals, and themaximumvalue of said proportional loss is small in comparison with the value ofsaid large loss inserted in the incoming one-way circuit atv said otherterminal by said second control device.

BJORN G. BJORNSON.

